Note: Descriptions are shown in the official language in which they were submitted.
This invention relates to slurry cast crosslinked double
base propellants having high burning rates, good pressure/rate
exponents (n), and producing little or no primary smoke.
Large double base propellant motors are presently pre-
pared by ~wo processes, conventional or "in situ" casting and"slurry casting". Each of these processes has advantages
depending on the desired properties of the final propellant.
In the "in situ" casting process, casting powder granules
comprised of precolloided nitrocellulose are charyecl to a mold
and covered with a casting li~uid. The casting powder gran-
ules are generally comprised of nitrocellulose, nitroglycerin,
ammonium perchlorate, solid nitramine, stastabilizer and bal-
listic modifiers. The casting liquid is typically comprised
of an explosive li~uid such as nitroglycerin and a nonexplo-
sive plasticizer such as triacetin or dibutylphthalate. Thenitrocellulose portion of the casting powder granules absorbs
the casting liquid and swells to form a consolidated mass.
In the present crosslinked double base (XhDB~ slurry
casting process a pourable slurry of propellant ingredients is
prepared and the slurry is cast into a mold and cured. The
slurry contains nitrocellulose in solution with other polyols
which are crosslinked with polyisocyanates during cure to form
a solid propellant. Fine solid oxidizers such as cyclotetra-
methylenetetranitramine (HMX) are present in the slurry to
increase propellant energy and to reinforce the binder system
and thereby improve mechanical properties.
Advantages of the slurry casting process over "in situ"
; methods are relatively short cure times, low processing costs,
improved strain capability at low temperatures, increased
performance and adaptability of the process to include highly
energetic and sensitive propellant ingredients in the
li~2
--2--
propellant formulation because of relatively mild mixing
conditions oE the slurry process.
To increase the burning rate of smokeless crosslinked
double base propellants prepared by the slurry casting pro-
cess, ballistic modifiers (e.g., metal compounds such as Pb2O3and SnO2 or lead salicylate and lead beta-resorcylate with
carbon black) have been added to the propellant matrix during
processing. These metal compvunds also aid in maintaining a
satisfactory pressure/rate exponent ~n)~ Maximum burning
rates of about 0.5 in./sec. at 1000 psi with n values of 0.4
to 0.6 are obtained by using relatively large amounts (2-4%
based on the weight o~ the propellant composition) of these
ballistic modifiers.
Ano~her method of increasing burning rate o~ double ba~e
crosslinked propellants involves the addition ,of rine ammonium
perchlorate to the propellant matrix. When using this method,
however, large quantities of ammonium perchlorate are required
to boost burning rate. For example, burning rates of 0.37,
0.35, and 0.54 in./sec., respectively, at 1000 psi were ob-
tained when 0, 5%, and 15% of 6 microns ammonium perchloratewas added at the expense of cyclotetramethylenetetranitramine
(~MX) in preparation of crosslinked double base propellants
that used Pb~03/ SnO2, and carbon black as ballistic modi-
fiers. A decrease in ammonium perchlorate particle size does
not significantly increase burniny rate when ammonium per-
chlorate is present in moderate concentrations; burning rates
were 0.34 and 0.55 in./sec., respectively, at 1000 psi when
5~ and 10% of 2 microns ammonium perchlorate was employed in
the propellant matrix. Additional increases in burning rate
have been obtained by further increases in ammonium perchlor-
ate content but penalties involving increased impact and
friction sensitivity and increased secondary smoke formation
have been incurred.
U.S. Patent 4,080,411 describes a slurry casting process
in which ~lake casting powder is combined with explosive and
nonexplosive plasticizers, a polyglycol adipate-tolylene
diisocyanate prepolymer, a stabilizer and a solid nitramine to
prepare solid propellants. The flake casting powder described
~ 6
--3--
in U.S. Patent 4,080,411 comprises nitrocellulose and is
prepared by the solvent process for manufacture of casting
powder. In the solvent process a viscous propellant mass com-
prising nitrocellulose and solvent is pressed into a block and
extruded into small strands of propellant of circular cross-
section. These small strands are cut into flakes, dried to
remove all solvent, glazed with powdered graphite and screened
to proper size. In the process described, uniform distribu
tion of ballistic modifiers such as lead beta-resorcylate and
lead salicylate in the flake casting powder is readily accom-
plished by adding such ballistic modifiers to the viscous
propellant mass prior to blocking and extrusion.
U.S. Patent 3,813,45~ describes a slurry casting process
for manufacture of double base propellant in which metallic
staple is distributed throughout the propellant to increase
the propellant burning rate. The slurry casting process of
U.S. Patent 3,813,458 employs both casting powder granules
containing ammoniumn perchlorate and densified (plastisol)
nitrocellulose as two sources of nitrocellulose in the pro-
pellant. The casting powder granules are primarily included
within the slurry to for~ interstices throughout the slurry
which interstices are intended to trap metallic staple in the
mixing process to provide uniform distribution of metallic
staple throughout the propellant.
In accordance with this invention, a high specific im-
pulse crosslinked double base propellant having a high burning
rate is prepared by a slurry casting process comprising form-
ing a propellant slurry comprising nitrocellulose, explosive
plasticizer, curing agents and oxidizers. Casting powder
granules are admixed with said slurry , said casting powder
granules comprising from about 10% to about 25% by weight
based on the weight of the propellant slurry of a double base
casting powder composition which contains from about 20~ to
about 75% by weight of ammonium perchlorate particles having
a particle size range of from about 0.5 micron to about 3.0
microns. I'he resulting propellant composition of this inven-
tion is cast and cured. Upon curing, the propellant contains
casting powder particles which substantially retain their
. .
r-L~
~4--
identity as granules except for a slight absorption of plasti-
cizer and curing agent which results in a slight size increase
of the granules and except for a reaction of nitrocellulose in
the propellant granule with the curing agents employed in the
slurry. Because of the mixing action imparted in preparing
the slurry/ discrete par~icles o~ casting powder having a high
burning rate are uniformly dispersed throughout the
propellant.
The castin~ powder granules which are employed in prepar-
ing the propellant composition of this invention are of the
double base type (contain nitrocellulose and explosive plasti-
cizer) containing fine particle ammonium perchlorate and are
prepared in the usual manner employed in smokeless powder man-
ufacture save that anhydrous conditions and non-solvents for
ammonium perchlorate are used to prevent ammonium perchlorate
growth during processing. Anhydrous conditions are achieved
by removal of trace quantities of moisture in nitrocellulose
by azeotropic distillation of nitrocellulose with hexane.
Ethyl acetate and hexane are used as processing solvents
because they are not solvents for ammonium perchlorate. The
amount of fine particle ammonium perchlorate employed is
limited by factors involving processability and sensitivity.
The casting powder granules can contain from about 20% to
about 75% by weight of ammonium perchlorate. The particle
size of the ammonium perchlorate is from about 0.5 micron to
about 3.0 microns. The maximum amount of ammonium perchlorate
solids which can be readily incorporated in the casting powder
granules will decrease with decreasing ammonium perchlorate
particle size~ Thus, up to about 75% by weight o~ ammonium
perchlorate solids having a 2.0 micron particle size can be
employed in casting powder granules. Above about 75%,
; processability becomes extremely difficult. When employing
0.5 micron ammonium perchlorate in a casting powder granule,
about 65~ by weight of such ammonium perchlorate is the
maximum amount which can be processed. The casting powder
granules are employed in amounts of from about 10~ to about
25% by weight of the propellant composition.
--5--
The casting powder granules are preferably as small as
is reasonably practicable, preferably the diameter and length
measurements of the casting powder granules being approxi-
mately equal. The length and diameter of the casting powder
granules may range from about 10 mils to about 50 mils but
preferably the length and diameter of the granules are about
30 mils each or less.
The nitrocellulose component of the casting powder gran-
ules is preferably nitrocellulose having a nitrogen content of
12.6% N and having a 10-20 second viscosity as measured by a
falling ball method (MIL-N-244A) employing a solution compris-
ing 10% nitrocellulose, 10% denatured alcohol and 80% acetone.
Other grades of nitrocellulose which can also be used include
those with nitrogen contents of 11.8% to 13.4% and viscositie~
of 18 centipoise (cp) to about 6000 seconds. The viscosity of
13.4~ N nitrocellulose is determined using the military speci-
- fication (MIL-N-244A). Viscosities of other nitrocellulose
types are determined using another falling ball method defined
in ASTM D 301-56 employing a solution comprising 25% denatured
alcohol, 55% toluene and 20% ethyl acetate. The nitrocellu-
lose concentration employed in determining viscosity varies
with the type nitrocellulose being tested. Such concentra-
tions are 12.2% for 5 second or higher nitrocellulose, 20% for
1/2 and 3/4 second nitrocellulose and 25~ for 18-25 cp, 30-35
cp, 1/4 second and 3/8 second nitrocelluloses. Nitrocellulose
comprises from about 5~ to about 40% by weight of the casting
powder granula.
Plasticizers employed in preparations of the casting
powder granules are explosive liquids such as nitroglycerin,
butanetriol trinitrate, trimetriol trinitrate, and the like.
These plasticizers are employed in the casting powder granules
in amounts of from about 10% to about 40~ by weight.
In addition to ammonium perchlorate, nitrocellulose and
plasticizer, additional ingredients such as other polyols,
aluminum oxide, ballistic modifiers, graphite linters, alumi-
num or zirconium staples, carbon black and various stabiizers
can be included in the casting powder granules. The concen-
tration ranges of these optional inyredients (weight percent)
f~
which can be employed are set forth in Table I below.
Table I
Polyol (other than Nitrocellulose) 0-40%
Al23 0-3%
Ballistic Modifiers 0-4%
Graphite Linters or Metal Staples 0-5%
Carbon Black (powder) 0-0.5~
Stabilizers 1-3.0%
The initial slurry of the propellant, i.e., th~ propel-
lant slurry excluding the casting powder granules, is prepared
from nitrocellulose, polyol, curing agent, organic oxidizers,
explosive plasticizers, stabilizers and minor amounts of other
ingredients. The preferred nitrocellulose is low viscosity
nitrocellulose containing 12% ~ and having a viscosity of 18
cps-25 cps (measured at 25C. using 25~ nitrocellulose and a
solvent containing 25% ethanol, 55% toluene and 20% ethyl-
acetate? and an approximate intrinsic viscosity of 0.4
deciliters/gram (determined using acetone solvent). Other
nitrocellulose types with viscosities of up to 5 seconds can
also be used. Viscosity of 5 second nitrocellulose is
determined using the above solvent at 12.2% nitrocellulose
concentration.
Polyols that can be employed in the initial slurry of
the propellant are polyester polyols, polyethylene glycols,
poly(oxyethylene-butylene) glycols, and polycaprolactones.
The polyols employed generally have a molecular weight range
of from about 2000 to about 6000 and a hydroxyl functionality
of from about 2 to about 3. Polyester diols which can be
employed can be prepared by reaction of monomeric dialcohols
such as ethylene glycol, diethylene glycol, propylene glycol,
butylene glycol hexamethylene glycol, mixtures thereof and
the like, with dibasic acids such as adipic acid, succinic
acid, azelaic acid, sebacic acid, oxadibutyric acid, mixtures
thereof, and the like. Polyglycol adipate is a preferred
polyol to be employed with nitrocellulose in the initial
slurry of the propellant.
The combination oE nitrocellulose and polyols comprises
Erom about 5% to about 12% by weight of the initial propellant
slurry. The nitrocellulose content can be from 0~ to about 4%
~f~ 6V
and the polyol from about 1~ to about 12~. The preferred
initial slurry contains from a~out 0.4% to 2.0% nitrocellulose
and from about 4% to about 7% polyol, preferably polyglycol
adipate.
Plasticizers employed in preparations of the initial
slurry are explosive liquids such as nitroglycerin, butane-
triol trinitrate, trimetriol trinitrate, and the like. These
plasticizers are employed in the initial slurry in amounts of
from about 5% to about 50~ by wei-ght.
Polyfunctional isocyanates are employed as curing agents
for the nitrocellulose and polyols which form the binder of
the propellant compositions of this invention. The polyfunc-
tional isocyanates which can be employed have an NCO function-
ality of two or more. Illustrative polyfunctional isocyanates
which can be employed include tolylene diisocyanate, hexa-
methylene diisocyanate, 3-isocyanatomethyl-3,5,5-trimethyl-
cyclohexaneisocyanate, isocyanates having a functionality Orc
3 or more prepared by reaction of diisocyanates such as hexa-
methylene diisocyanates and water, and the like. Curing
agents are generally employed in amounts of from about 0O8%
to about 2% based on the weight of the initial slurry, but
greater and smaller concentrations can be employed. In addi-
tion to the polyfunctional isocyanate curing agent, a curing
catalyst is preferably employed to speed the rate of cure of
the propellant by catalyzing the reaction of isocyanate groups
with hydroxyl groups. Il~ustrative curing catalysts include
triphenyl bismuth (TPB), dibutyl tin diacetate (DBTDA), and
dibutyl tin diluarate.
Organic oxidizer solids can be employed in the propellant
slurry. Illustrative organic o~idizers include cyclotetra-
methylenetetranitramine (HMX); cyclotrimethylenetrinitramine
(RDXl; solid nitramines such as 2,5-dinitrazahexane and solid
nitro compounds such as hexanitrostilbene and nitroguanidine.
The solid organic oxidizers employed must have a small par-
ticle size so that they can be dispersed readily through thepropellant mass ancl remain uniformly dispersed after mixing
has been completed. Organic oxidizers comprise from about
l~Ci~
30% to about 50% by weight of the initial slurry/ but greater
and smaller concentrations can be employed.
The following examples further illustrate this invention.
In the examples and throughout this specification, percentages
are by weight unless specified otherwise.
Exam~les 1-4
Casting powder granules employed in the slurry process of
this invention are prepared as follows:
Hexane~wet nitrocellulose, nitroglycerin, ammonium per-
chlorate, stabilizer, ultrafine carbon black and 1/2 inchlength graphite linters are mixed in a sigma blade mixer using
a mixture of hexane and ethyl acetate as processing solvents
in sufficient quantities to produce a propellant dough. The
resulting dough is pressed through 30 mil dies to obtain
strands which are cut into 27 mil length casting powder.
Residual solvents are removed by oven drying to obtain casting
powder, composi~ion A. Composi~ion ~ is prepared in the same
manner as composition A blt it contains A12O3~ The casting
powder compositions are se~ forth in Table II below.
Table II
Castin~_~owder
Composition (~) A B
Nitrocellulose 29.00 28.00
(12.6%, 10 sec.)
Nitroglycerin 25.24 25.24
Stabilizer 2.76 2.76
Ammonium perchlorate (2 ) 40.00 40.00
Graphite linters(a) 3.00 3.00
A123 1.00
(a) Graphite linters, nominal diameter of 10 , Type HMS
manufactured ~y Hercules Incorporated.
Slurries containing casting powder granu]~es of composi-
tions A and B are prepared by forming an initial slurry by
mixing of ingredients, i e.l a lacquer containing 18-~5 cp
nitrocellulose and nitroglycerin, additional nitroglycerin,
stabilizers, a polyglycol adipate, small particle sized HMX
and cure catalysts (TPB and DBTDA) at reduced pressure ( 15 mm
Hg) adding hexamethylene diisocyanate curing agent and then
adding casting powder granules to the slurry and mixing again
- 9 -
at the reduced pressure of 15 mm Hg. Additional ballistic
modifiers are also added to the propellant slurry in preparing
the propellant composition of Example 2. The initial propel-
lant slurries, i.e., prior to addition of the casting powder
granules have the compositions set forth in Table III:
Table III
-
Example No. 1 2 3 4
Casting Powder Comp. Comp. Comp. Comp.
"A" "A" "B" "A"
Slurry
Composition
(parts)
Nitrocellulose,1.29 1.29 1.29 1.29
12% N (18-25 cp)
Nitroglycerin 35.92 35.92 35.92 35.92
Polyglycol 5.46 5.38 5.51 5.~3
adipate
Stabilizer-l 0.86 0.86 0.86 0.84
Stabilizer-2 0.36 0.36 0.36 0.3~
Hexamethylene 0.99 1.07 0.94 0.81
diisocyanate
HMX (5 microns)40.12 38.25 40.12 37.76
SnO2 - 0.85 - _
Pb23 - 0.85 - _
3~ Carbon black(a) _ 0.17
TPB (added)(b) 0.02 0.0~ 0.02 0.02
DBTDA X104 (c) 8 8 8 7
(a) Ultrafine carbon black having a surface
area of 1125 m2/gram.
(b) TPB is triphenyl bismuth.
(c) DBTDA is dibutyl tin diacetate.
The resulting slurries to which the casting powder has
been added ar~ each mixed for 15-20 minutes at 100-110F. at
reduced pressure. The propellants are then cast into molds
and cured for 4 days at 50-70F. and 7 days at 120F. The
initial low temperature cure is used to permit some exchange
of plasticizers between slurry and casting powder and to per-
mit some absorption of curing agent into the casting powder
while the 120F. cure is used primarily to allow curing agent
to react with functional hydroxyl groups in nitrocellulose
and polyglycol adipate and thereby solidify the propellant.
--10--
The composition of each of the resulting propellants is given
in Table IV.
Table IV
Ex.l Ex.2 E~.3 Ex.4
Composition
Nitrocellulose 4.35 4.35 4u205.80
(12.6~ N, 10 sec.)
Nitrocellulose 1.29 1.29 1.291.22
~ 25 cp)
Polyglycol 0 5.38 0 0
adipate (a)
Polyglycol 5.46 0 5.515.23
adipate (b)
Nitroglycerin 39.70 39.70 39.7038.85
Stabilizer 0.55 0.55 0.550.59
Stabilizer 0.94 0.94 0.940.91
Stabilizer 0.15 0.15 0.150.20
Hexamethylene 0.99 1.07 0.940.84
diisocyanate
HMX ~5 microns) 40.12 38.25 40.1237.76
Ammonium 6.00 6.00 6.008.00
perchlorate (2 microns)
Graphite hinters (c) 0.45 G.45 0.45 0.60
Carbon Bla-ck (d) 0 0.17 0 0
SnO2 0 0.85 0 0
Pb23 0 0O85 0 0
A12O3 (0.1 micron) 0 0 0.15 0
(a) Polyglycol adipate, hydroxyl functionality of
about 2.7, molecular weight of about 2400.
(b) Polyglycol adipate, hydroxyl functionality of
about 2.7, molecular weight of about 4,000.
(c) See footnote (a), Table II.
(d) Ultra~ine carbon black having a surface area
of about 1125 m2/gram.
; 35 The propellant compositions of Examples 1-4 were evalu-
ated for rheological properties, ballistic data and mechanical
properties. Results of these evaluation tests are set orth
in Table V.
..,
~5
Table V
Ex.l Ex.2 Ex.3 Ex.4
Rheolo~ical Properties
Viscosity ~kp~(a) 3.7 3.4 4.0 4.2
n~(b) 0.39 0.45 0.47 0.56
Pot Life (hours~ 4~ (c) (c) 5
Strand Ballistic Data
. .
rlooo (in./sec.~ 0.87 0.95 0O94 1.05
n from 500-1000 p~i 0.65 0.61 0.77 0.69
n from 1000-1500 psi 0.65 0.61 0.50 0.69
n from 1500-2000 psi 0.78 0.61 - 0.69
Mechanical Properties at 77F. at 0.74 in./in./min. (Instron
Tester)
Tensile 76 88 78 92
Strength (psi)
Elongation ~) 26 42 39 30
Modulus (psi~ 440 360 340 540
(a) Viscosity at 32C. at 0.57 sec.~l
(b) Flow behavior index (dimensionless)
~0 (c) Not determined
Of the foregoing examples, the propellants of Examples 1
and 4 illustrate preferred embodiments because their compo-
sitions do not contain metal oxides as do the propellants of
Examples 2 and 3. The compositions of Examples 1, 3 and 4 do
not produce primary smoke upon combustion. Primary smoke is
visible because of the presence of particles of metal reaction
products in the propellant exhaust. Lead compounds which
further augment the burning rate of the propellant composi-
tions of this invention react during combustion forming lead
chloride which augments smoke visibility. In order to main-
tain smokelessness, metals are preferably omltted from the
propellant compositions of this invention or employed in small
amounts, say of less than 2~ by weight of the propellant
composition.
Secondary smoke is a contrail of visible water crystals
or droplets which form as a result oE the hygroscopic action
of hydrogen chloride in the exhaust with water vapor. The
visibility of secondary smoke is increased by decreasing
temperature or by increasing relative humidity. Propellants
-12-
containing ammonium perchlorate produce some secondary smoke.
The propellants of this invention exhibit high burning rates
but employ comparatively low amounts of ammonium perchlorate
and, therefore, produce low amounts of secondary smoke. Com-
posite propellants are comprised almost entirely of ammoniumperchlorate and secondary smoke evolution of composite
propellants is high.
The propellant compositions of this invention have a rel-
atively high specific impulse largely because of the presence
of a relatively large amount o solid organizer, such as
cyclotetramethylene tetranitramine (HMX) in the propellant.
High burning rates can be obtained despite the presence of the
large amounts of the solid organic oxidizers such as HMX which
tend to depress the burning rate. Burning rate is high be-
cause of the relatively high concentration of ammoniump~rchlorate in discrete areas which are uniformly distributed
throughout the propellant.
Propellants of this invention have advantages over pro-
pellants prepared by a slurry casting process containing fIake
- 20 casting powder. Burning rates are higher and pot life is
longer for propellants of this invention. Propellant slurries
of this invention can have a pot life in excess of ive hours
; compared to a 20-60 minute pot life or slurries containing
flake casting powder.
